Rolling lines are known for the production of metal strip starting from the continuous casting of slabs. Such lines normally provide a continuous casting, a heating and/or maintenance furnace, a possible cropping shear, a reversing or continuous rolling train, a cooling system and one or more winding units to form coils of the desired weight.
In particular, for example from U.S. Pat. No. 4,675,974, U.S. Pat. No. 5,396,695 and U.S. Pat. No. 6,182,490, a rolling line is known in which the continuously cast slab, after being sheared to size to produce a coil, is sent to the tunnel furnace, of a length at least equal to the segment of slab, which homogenizes the temperature, passing to a reduced speed, until the temperature is brought to suitable values for subsequent workings.
The segment of slab exiting from the tunnel furnace is accelerated and sent to the rolling unit.
It is known that rolling plants with a reversing rolling train of the Steckel type with one or more stands, an example of which is schematically shown in FIG. 1, use a slab with a thickness from 150 to 250 mm or more, and work with coil to coil mode, that is, with a length of slab which in relation to the thickness is equal in weight to a coil of finished product. In such plants there is a productivity limit, of a minimum final thickness, which in general is never less than 1.8-1.6 mm, and of a dimensional and surface quality of the strip: the productivity is limited by the high number of inversions and passes through the stand or stands, and by the connected down-times; the final minimum thickness is limited by the great thickness of the slab at inlet; and the dimensional and surface quality is limited by the great difference in temperature between head/tail and the central part of the strip.
Moreover, the reversing Steckel rolling mill creates a problem connected to the fact that in the first rolling passes, the roughed slab, the so-called “transfer bar” or simply “bar”, cannot normally be immediately wound in the reel furnaces disposed upstream and downstream of the stand, because of the great thickness of the entering slab, thus creating a problem of bulk of the line as the length of the slab increases.
Moreover, the great number of rolling passes, with consequent winding and unwinding in the reel furnaces placed upstream and downstream of the stand/stands, induces a cooling at the tip and the tail, as well as uneven temperatures along the coil which penalizes the yield because of the need to carry out head and tail cropping.
The high number of passes also determines variable dimensional tolerances in length and limitations in the production of thin thicknesses, and also rapid wear on the work rolls due to the high number of passes and the low temperature of the material being rolled and the leading/tail ends.
The entry of the cold and deformed leading ends into the furnaces upstream and downstream of the stand/stands is a delicate operation, with the risk of jamming, which becomes more and more probable with the reduction of the thickness of the strip.
One purpose of the present invention is to perfect a rolling method, and achieve a relative line, for the production of flat products in so-called semi-endless mode, which allows increase productivity, to increase the yield with respect to known plants and processes and which allows to obtain very thin thicknesses, from 1.0 to 2.0 mm, even as little as 0.8 mm.
Another purpose is to reduce the problem of jamming and blockages, in particular in the reel furnaces and in the coilers which form the final coil, even in the production of very thin thicknesses, below 2.0-2.5 mm, in any case maintaining high productivity and quality of the final product, irrespective of the type of steel cast.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.